Close, but not quite. In this case, the maximum employer contribution would be 20% of net earnings. Net earnings are net profit minus the deductible portion of self-employment taxes. If you do not have a substantial amount of other income subject to social security taxes, the maximum employer contribution based on $50,000 of net profit would be $9,293 for a maximum total contribution of $35,293.
Assuming that neither spouse has any interest in the other spouse's business, each business must have it's own 401(k) plan and contributions would be based entirely on the income associated with that spouse's business. Yes, you would need to obtain an EIN for the business and use that when establishing the solo 401(k).
The deadline for establishing a 401(k) plan used to be December 31 of the year for which contributions were to be made, but the SECURE Act changed that to be the filing deadline for the tax return for that year. So, if you are a sole proprietor, you would have until April 15, 2021 to establish a 401(k) to receive contributions for 2020, or October 15, 2021 if you request a filing extension.
